To increase 80 by 15%, find 15% of 80 (12) and add it: 80 + 12 = 92. To decrease 80 by 15%, subtract instead: 80 − 12 = 68.
A quicker way uses a multiplier. Increasing by 15% means you end with 115% of the amount, so multiply by 1.15: 80 × 1.15 = 92. Decreasing by 15% leaves 85%, so multiply by 0.85: 80 × 0.85 = 68. Multipliers make repeated changes easy.
Worked example
A price of $240 rises by 20%. What is the new price?
- 20% of 240 = 48.
- 240 + 48 = 288. Or 240 × 1.2 = 288.
Answer:$288
Key words
- percentage increase
- an amount gets bigger by a percentage
- percentage decrease
- an amount gets smaller by a percentage
- multiplier
- the decimal you multiply by, like 1.15
Practise this
New questions every time.
Common Core-referenced: 7.RP.A.3